A current system for tracking position of a survey instrument or tool, such as the Spectra Precision® Optical TS415 Total Station from Trimble, consists of a fixed ground station and a reflector, which is usually mounted onto a pole. The pole creates a vertical offset from the surface of the region being surveyed. A leveled self-tracking laser theodolite on the ground station sends out a laser that hits the reflector and is bounced back to the ground station. A data processor of the tracking system calculates the x, y, z, and position of the reflector by measuring the time of flight for the bounced laser and the two angles between the ground station and the reflector. The data processor compensates for any vertical offset between the reflector and the target. The data processor, however, does not correct for angular offsets. The surveying pole must be held completely vertical over the target. Most surveying poles are equipped with a “bulls-eye” level to aid in holding the pole completely vertical. This technique is adequate for survey applications that can utilize the vertical pole and reflector, for measurements of relatively fixed positions.
However, there are many applications where it is not feasible or does not make sense to have the pole vertical to the measured target. One such application would be tracking a moving vehicle with an attached sensor platform (survey tool) that has a rigidly attached survey pole, as the vehicle moves over an irregular surface. Often, ground or street surfaces, for example, have non-zero angles with respect to the horizontal (that is to say non-zero inclination). As the vehicle moves over such a surface, the pole remains perpendicular relative to the vehicle, typically with respect to the surface; but it will not always be vertical. In such an application, the tracking system would record erroneous position data whenever the vehicle encountered a non-horizontal surface.
In a similar fashion as described above, a GPS receiver could be employed to track a survey tool. And similarly, if the GPS receiver is mounted on a pole or housing that is rigidly affixed to the survey tool with some vertical offset from the surface, the GPS receiver would record erroneous position data whenever the vehicle encountered a non-horizontal surface.
A method to try and compensate for the non-horizontal surface effects would be to gimble-mount the survey pole (or GPS antenna). However, this still would not solve the problem. Although gimble mounting would keep the survey pole (or GPS antenna) vertical, the position of the bottom of the pole would always be changing relative to the survey tool as the contour of the ground or other surface varied.